Tree management: urban trees

Every tree on record

21 June 2011

Forget laborious fieldwork; now auditing the number, positions and potential impacts of urban trees is a matter for digital aerial photography and desk-based data analysis, says James Eddy

The furore over the UK government’s proposals for private ownership of forests and woodlands demonstrates the importance people attach to trees – and not just in the countryside. Trees and hedges have a major part to play in urban areas, aesthetically of course, but also in terms of providing wildlife habitats, improving the air we breathe and helping to conserve energy in nearby buildings. Studies in the US have also established a number of measurable benefits of urban vegetation, including a willingness to pay more for parking and goods in landscaped business districts, a positive impact on house prices and even reduced crime levels.

However, urban trees can cause problems too. Subsidence of houses is estimated to cost the UK insurance industry more than £500m after each dry year, and is usually the second most expensive insured peril after fire. In the majority of cases, particularly in areas of clay soil, trees are the main cause of subsidence. They can also be a major worry to homeowners when their position or condition threatens to jeopardise house sales and/or mortgage loans, or causes disputes with neighbours.

Clearly, it is vitally important that this valuable resource is properly managed and, where necessary, protected. Local authorities are responsible for managing trees in public spaces and on council land and they can protect them through tree preservation orders (TPOs) and conservation areas. Yet, while we are blessed with millions of trees in our cities and towns, very little information is available about them.

Collating data for a tree audit can involve hours of labour-intensive fieldwork. To be meaningful or useful, data must include not just the precise location of trees, but measurements of heights and canopies. Information about species, health and proximity to other urban features makes the data relevant to utility and insurance companies, emergency services and property developers and architects.

Hence, the development of what is thought to be the first digital map layer accurately depicting the location and extent of trees and their proximity to buildings. ProximiTREE was designed by aerial survey company Bluesky for use in geographical information systems (GIS), web-mapping applications and computer-aided design (CAD) packages. The data details the exact spatial location and height of each individual tree, plus the circumference of its canopy, from which can be determined the extent of root growth and thus, the potential impact of a tree on existing or proposed properties, roads or other trees.

Technically speaking

A team of editors specially trained in aerial photo interpretation carefully examines high-resolution aerial survey images to identify and then record the spatial location of individual urban trees. Using specialist software and the stereoscopic images, the heights of each crown and canopy are measured and then a digital terrain model (digital representation of the ground surface topography or terrain) is used to determine the real-world tree heights. Since the data includes both location and height measurements, it can be viewed and interrogated in both 2D and 3D GIS and CAD software.

Red Rose Forest, the community forest for central and western Greater Manchester, recently took delivery of the first ProximiTREE dataset. Red Rose works with local communities, businesses and partners to develop well-wooded, multi-purpose landscapes, and it headed a consortium including the Homes and Communities Agency and the University of Salford, to commission the map. The intention was to help authorities across Manchester understand the impact of urban trees on the environment, public health and the aesthetic qualities of the region. Red Rose had done tree audits before using basic aerial photography, which had shown the level of tree cover in the area was higher than expected, but uneven. Now they wanted much more detail.

When it comes to calculating the potential impact of trees, analysts can use a GIS or other software package to compare digital tree maps with other types of spatial information, such as existing vector maps from Ordnance Survey, address databases such as the National Land and Property Gazetteer, aerial photography and other specially collected data.

Further developments are planned, and work is already in progress on the automatic derivation of species and the possible inclusion of health indicators derived from complementary datasets such as colour infrared (CIR) data. Other complementary datasets such as detailed 3D height models can also be used to provide additional information and context for the tree data.